Fix that pgpool_pgctl can be executed only when the login user is super user in backe...

Fix that pgpool_pgctl can be executed only when the login user is super user in backend nodes.
Previously, any login user was able to try to execute pgpool_pgctl()
which could stop/restart/reload a backend node even if he was not a super user.
Also pgpool_pgctl() was executed by sr_check_user or health_check_user, but it wasn't reasonable.
This update is that buttons of stop/restart/reload are disabled when the login user isn't a super user
and that pgpool_pg_ctl() is executed by login user instead of sr_check_user or health_check_user
who don't have to be non-superuser.
Per [pgpool-general: 2338].